This invention relates to thermosettable bisimide compositions. In one aspect, the invention relates to the preparation of cured bismaleimide resins having enhanced fracture toughness and processability.
Advanced composites are high-performance materials made up of a fiber-reinforced thermoplastic or thermosettable material. The thermosettable materials useful in advanced composites must meet a set of demanding property requirements. For example, such materials optimally have good high-temperature properties such as high (above 200.degree. C.) cured glass transition temperature and relatively low (less than 4%) water absorbance at elevated temperature. Such materials must also exhibit high mechanical strength, as measured by fracture toughness and compression after impact. For ease of processing in preparing prepregs for composite parts, the uncured material will ideally have a low (below 120.degree. C.) melting temperature and a wide temperature range of processable viscosity (a wide "processing window").
Bismaleimide resins are thermosettable materials with much promise for use in advanced composites. Disadvantages of bismaleimides, however, include brittleness and high melting points, the latter of which requires bismaleimides to be used with solvents for acceptable processability.
Standard modifiers for bismaleimides have a number of disadvantages. Some, such as styrene, divinylbenzene, diisopropenylbenzene and certain derivatives thereof, are extremely reactive with maleimide groups by both Diels-Alder and radical mechanisms. This leads to a very short pot life for bismaleimide mixtures with these reactive diluents. These materials, while providing more easily processable bismaleimide resins, tend to be ineffective as tougheners for the resins. Diamines and dithiols generally share this disadvantage of high reactivity, and also the adducts of maleimide groups with amines or thiols are of somewhat low thermal stability. Other diluent modifiers, such as bisallyl, bis(allyloxy) or bispropenyl aromatic compounds, including dialyl bisphenol-A and adducts of allylphenols with epoxy resins, are somewhat less reactive than the vinyl aromatics but still have a pot life in the melt which is relatively short for many types of liquid processing applications.
It is therefore an object of the invention to provide bisimide resin compositions having enhanced physical properties and good processability for composites applications.